In addition to the permeability of the superabsorbents (SAPs) (reported in the form of what is called the “Saline Flow Conductivity—SFC”) and the absorption capacity under compressive stress, the absorption rate of the superabsorbent particles in particular (reported in amount of liquid absorbed per gram of superabsorbent per second) is also a crucial criterion which enables statements about whether an absorbent core which comprises this superabsorbent in a large concentration and has only a low fluff content is capable, on its first contact with liquids, of absorbing them rapidly (called “acquisition”). In the case of absorbent cores with a high superabsorbent content, this “acquisition” depends, among other factors, on the absorption rate of the superabsorbent material.
From the prior art, there are various known property rights which are supposed to enable an increase in the absorption rate of superabsorbent particles. WO96/17884A1 discloses a water-absorbing resin for which a solid blowing agent with a particle diameter of 1 to 100 μm is used in the monomer solution. In principle, preference is given to organic azo compounds and here specifically to the acrylic salts of azo compounds containing an amino group. Pure carbonates, ammonium nitride or mixtures thereof can optionally be used.
Disadvantages here are the rapid conversion of the azo compounds and the basic dispersion of the small solid particles in the monomer solution. Larger particles cannot be dispersed well without separation of particles and monomer solution in the dispersion before it reaches the gel point.
A disadvantage here in the case of use of superabsorbents known from the prior art is that leakage problems occur, since the SAP either absorbs the liquid too slowly and/or has unsuitable liquid transportation.
The current trend particularly in diaper construction is to produce even thinner absorbent cores with reduced cellulose fiber content and increased superabsorbent content. The advantage of thinner constructions is exhibited not just in improved wear comfort but also in reduced costs in packaging and storage. The newest generation of absorbent cores, which is described, for example, in WO-A-2008/155722, WO-A-2008/155711, WO-A-2008/155710, WO-A-2008/155702, WO-A-2008/155701, WO-A-2008/155699, EP-A-1 225 857, WO-A-01/15647, WO-A-2011/120504, DE-A-10 2009 049 450, WO-A-2008/117109, WO-A-97/11659, EP-A-0 826 349, WO-A-98/37846, WO-A-95/11653, WO-A-95/11651, WO-A-95/11652, WO-A-95/11654, WO-A-2004/071363 or WO-A-01/89439, is essentially cellulose-free (which is why corresponding diapers are also referred to as “fluffless diapers”). The immobilization of the superabsorbent particles, which in cellulose-containing absorbent cores is effected by the cellulose fibers, can be achieved in this newest generation of absorbent cores by, for example, immobilizing the superabsorbent particles on a substrate surface by means of thermoplastic fibers.
With the trend toward ever thinner diaper constructions and the omission of the temporary liquid storage and conduction function of the cellulose fibers, the profile of requirements on the superabsorbents has changed significantly. A factor of crucial importance is now the ability of the hydrogel to prevent the leakage of urine directly on micturition. This is achieved by the property of the superabsorbent/hydrogel of effectively absorbing the liquid during swelling and distributing it in the gel layer, with simultaneous minimization of the amount of unbound urine in the diaper. Due to good transport properties, advantageous superabsorbents also lead to optimal exploitation of the overall hygiene article.
U.S. Pat. No. 5,154,713 discloses water-absorbing polymers which are prepared by means of a carbonate blowing agent in the monomer solution. The carbonate particles are introduced here into the monomer solution well before the actual polymerization, and the initiator is added from 5 to 15 minutes after the dispersion of the carbonate blowing agent, as a result of which homogeneous distribution of these carbonate particles is not ensured and a not inconsiderable portion of the carbonate may be discharged again.
EP0644207 discloses superabsorbent polymers which are likewise admixed with an organic carbonate blowing agent in the monomer solution. Disadvantages here are the use of amine compounds, and also the elimination products of the organic carbonates remaining in the superabsorbent.
WO 2010/095427 discloses water-absorbing polymers in which a gas is dispersed into the monomer solution. This gas is nitrogen, argon, helium, carbon dioxide or the like, which is intended to ensure a more porous structure. The intention is to maintain these microbubbles in the monomer solution by means of polyoxyethylene-(20) sorbitan monostearate until the polymerization sets in. A disadvantage here is that the surfactants can be washed out of the end product again and adversely affect performance.
A factor of crucial importance is now the ability of the hydrogel to prevent the leakage of urine directly on micturition. This is achieved by the property of the superabsorbent/hydrogel of effectively absorbing the liquid during swelling and distributing it in the gel layer, with simultaneous minimization of the amount of unbound urine in the diaper. Due to good transport properties, advantageous superabsorbents also lead to optimal exploitation of the overall hygiene article.
The term “rewet” is generally understood to mean the property of a superabsorbent or of a composite comprising a superabsorbent to release liquid to an absorptive ply under compressive stress. The term “absorptive ply” is understood to mean, for example, paper, filter paper, collagen, sponges, foams or the like.
EP1858998B1 discloses superabsorbent foams where the monomer solution gives rise to foam only under an elevated pressure of 12 bar by addition of carbon dioxide and surfactants. However, the superabsorbents known to date from the prior art are only of inadequate suitability for use in the above-described new generation of cellulose-free diaper constructions.
In general, it is an object of the present invention to overcome the disadvantages arising from the prior art.
More particularly, it is an object of the present invention to provide a process for producing a water-absorbing polymer which has an improved swell rate and faster absorption of liquids, while simultaneously maintaining the overall quality, and more particularly a high permeability.
It is also a further object to perform the process in an economically simple manner, the intention being to minimize the use of organic additives, and also a mode of operation at ambient pressure.
It is a particular object of the present invention to provide a process by which water-absorbing polymers can be produced, and a particularly high swell rate can be ensured.
It is a further object of the present invention, in addition, to provide a process by which it is possible to produce water-absorbing polymers which ensure rapid and active liquid transportation, for example in thin diapers, such that rapid absorption and good distribution, i.e. corresponding capillarity, are ensured.
It is a further object of the invention, in particular, to specify a water-absorbing polymer, composites comprising such water-absorbing polymers, and chemical products comprising such water-absorbing polymers or composites, the water-absorbing polymers having an increased absorption capacity for aqueous solutions.